Luo Cuifen knew something was wrong when she went to the doctor after finding blood in her urine, but nothing could have prepared her for the discovery of 23 sewing needles, which doctors believe were stuck deep into her as a baby by her grandparents.

Doctors suspect they wanted to kill her because her family preferred a son. Some of the needles were pushed into the fontanelle, the soft spot on the head all babies have before the bones knit. Ms Cuifen, now 29, was a second granddaughter, leaving the family no chance to produce a treasured boy child.

Under the one-child policy, most families are limited to one offspring, but in the countryside, people may be allowed to have a second if their first is a girl.

The needles are about an inch long, and X-rays show they have worked their way into her lungs, liver, bladder and kidneys, making removal difficult, said Qu Rui, a spokesman for the Richland International Hospital in Kunming, the capital of Yunnan province. More than 20 surgeons and doctors from China, Canada and the United States, experts in disciplines from gynaecology toneurology, are to operate to try to remove the first six needles today.

Over the years, Ms Luo has suffered from inexplicable anxiety and insomnia, as well as depression. At times, the needles would emerge from strange wounds on her body. Ms Luo's grandparents are dead and there was no indication as to whether there would be an investigation.

There is a strong cultural bias in rural China against baby girls, sometimes known as "maggots in the rice". There is a saying that "raising a daughter is like watering someone else's fields", a sign of how deep-rooted the pro boy bias is. Boys carry on the family name but, for a girl, families need to find a dowry when she marries into someone else's family.

And having a son is the closest thing to a pension plan most rural Chinese will have: sons are supposedly better able to provide for the family and support their elderly parents. Because of the prejudice against girls, the increasingly mobile nature of Chinese society and the pressures of the one-child policy, many women do not report their pregnancies and if the child is a girl, they kill the baby.

Thousands of baby girls are abandoned every year, some on rubbish heaps. Others end up in the "dying rooms" of orphanages where they die of neglect, although the government has said it is cracking down on this.

There are 37 million more men than women in China, the most unbalanced gender ratio in the world. This skewed ratio has worsened since China introduced the one-child policy 30 years ago to curb population growth, making abortion a widely used method of familyplanning, and sometimes infanticide. The government reckons it has prevented 400 million births.

Official figures show there are 119 boys reported born for every 100 girls. The ratio in industrialised countries is 105 boys for every 100 girls. The gender gap raises the prospect of millions of men unable to find a wife, risking antisocial and violent behaviour. There are already 18 million more men than marriageable women. Gender scanning of a foetus is illegal in China but the regulation does not spell outpunishments, so a black market flourishes, with scans costing £3.40 if it is a boy and £2 for a girl.